The present invention relates to a semiconductor power conversion apparatus using a semiconductor device, and more particularly to a semiconductor power conversion apparatus that suppresses overvoltages caused by switching operations.
When an IGBT (Insulated Gate Bipolar Transistor) is used in a power conversion apparatus, energy stored in wires is applied as surge voltages to the IGBT during its turn-off. A method of preventing a destruction of the IGBT due to overvoltages such as surge voltages during the turn-off is disclosed, for example, in xe2x80x9cSeries Connection of Snubberless IGBTsxe2x80x9d, Proceedings of 2000 IEE Japan, Industrial Application Conference, FIG. 1. This conventional technique discloses an active gate control method which divides a collector voltage by resistors and, based on the potential at the dividing point, determines a gate voltage command value to suppress overvoltages.
This conventional technique, as shown in FIG. 2, connects a gate of IGBT 1 to a voltage dividing point through a buffer circuit so that the gate voltage of the IGBT 1 is set to a voltage at the voltage dividing point. When an on-off pulse generator 7 outputs a negative potential while the IGBT 1 is in xe2x80x9cONxe2x80x9d state, an electric charge accumulated at the gate of the IGBT is drawn out through a gate resistor 8, lowering the gate voltage, causing the IGBT to shift to the turn-off state and the collector voltage to rise. Even in a situation where the IGBT is applied surge voltages from the energy accumulated in leakage inductance of main wires, the conventional technique can cause a gate-emitter voltage (gate voltage) to rise, following an increase in the dividing point voltage resulting from an increase in the collector voltage. This in turn decreases the impedance of the IGBT 1 and thus clamps the collector voltage, protecting the device from an overvoltage destruction.
In the conventional technique described above, when the collector voltage of the IGBT 1 is divided by resistors, it is preferred that the voltage dividing resistors pass a larger current than the leakage current of the device and have a reduced loss. Hence, normally a resistor on the higher voltage side of the dividing point preferably has a resistance of 5-100 kxcexa9, and a resistor on the lower voltage side is set at or less than 1/20 the resistance of the higher voltage side resistor (gate dielectric strength/collector dielectric strength). The resistor on the higher voltage side, since it is applied a high voltage between its terminals and has a large thermal loss, uses a construction of a wire wound resistor as shown in FIG. 4 or a resistor having conductive particles 33 dispersed in an inorganic substance 34 as shown in FIG. 5.
The main conductive paths of the wire wound resistor shown in FIG. 4 and of the resistor shown in FIG. 5 with conductive particles scattered in an inorganic substance have parasitic capacitances. The wire wound resistor has a wire 31 with a large resistivity wound to produce a large resistance, but there is a stray capacitance (stray capacitance present in the main conductive path) 50 between its windings. When the resistance is high, the impedance due to an inductance component can be ignored. In the resistor shown in FIG. 5 having conductive particles 33 dispersed in an organic substance 34, too, there is a stray capacitance 50 in the main conductive path. Reference numeral 32 represents electrode terminals of the resistor.
Since the high-resistance resistor 3 on the higher voltage side has a stray capacitance between its terminals as described above, if the collector voltage is divided by a resistor 3 and a resistor 4 as shown in FIG. 2, this practically divides the collector voltage of the IGBT 1 by an equivalent circuit comprising the resistor 4 and a series-parallel circuit of resistors 38 and capacitors 50, as shown in FIG. 6. Thus, if a voltage rising rate (dv/dt) of the collector voltage of the IGBT 1 is large when the IGBT 1 is turned off, the impedance of the high-resistance voltage dividing resistor 3 on the higher voltage side decreases, raising the dividing point voltage and increasing the IGBT gate voltage more than necessary, which in turn lowers the IGBT impedance excessively and increases the turn-off loss.
It is therefore an object of the present invention to provide a semiconductor power conversion apparatus which suppresses a steep rise of the IGBT collector voltage to protect the IGBT against overvoltages, and which has means for preventing the collector voltage from being clamped excessively when the voltage rising rate (dv/dt) of the collector voltage becomes large and thereby preventing an increase in the turn-off loss.
To solve the problems described above requires fixing the case of the high-voltage side resistor to the emitter potential of the IGBT. That is, in one aspect, the present invention provides the semiconductor power conversion apparatus which comprises: a circuit for diving a collector voltage of an IGBT; and means for controlling a gate potential of the IGBT to a potential of a voltage dividing point in the collector voltage dividing circuit to protect the IGBT against an overvoltage applied to a collector of the IGBT; wherein a voltage of a case of a resistor on a high-voltage side of the voltage dividing point is fixed to an emitter potential of the IGBT, and a plurality of the IGBTs connected in series are switched simultaneously.
In another aspect, the present invention provides a semiconductor power conversion apparatus wherein the IGBT collector voltage dividing circuit has a high-voltage side resistor and a low-voltage side resistor, and wherein a sum of terminal-to-terminal resistances of the high-voltage side resistor and the low-voltage side resistor divided by the resistance of the low-voltage side resistor is equal to an impedance produced by a stray capacitance between the terminals of the high-voltage side resistor divided by an impedance produced by a stray capacitance between a high-voltage side terminal of the high-voltage side resistor and the case of the high-voltage side resistor.
As described earlier, in wire wound resistors such as those shown in FIG. 4 and in resistors having conductive particles scattered in an inorganic material as shown in FIG. 5, there are also stray capacitances between a main conductive path in the resistor and a case of the resistor. By fixing the case of the high-voltage side resistor as a conductor to the emitter potential of the IGBT, a current, which flows through the stray capacitance present in the main conductive circuit to the low-voltage side resistor, can be bypassed to the emitter of the IGBT through the stray capacitance present between the main conductive path and the resistor case. Hence, even when the voltage rising rate (dv/dt) of the IGBT collector voltage is large, it is possible to suppress an excessive rise of the potential of the voltage dividing point between the high- and low-voltage side resistors.
When the case of the high-voltage side resistor is fixed to the emitter potential of the IGBT, the IGBT and the resistors for dividing the IGBT collector voltage can be represented by an equivalent circuit shown in FIG. 7. Hence, if a ratio between the terminal-to-terminal impedance of the resistor and the impedance between the high-voltage side terminal of the resistor and the emitter is set equal to a ratio between the resistance component of the high-voltage side resistor and the resistance of the low-voltage side resistor, the collector voltage can be divided more accurately.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.